Bring the apple cider to a boil over med-high heat, then reduce the heat to keep the cider at a simmer (you should see small bubbles as the cider simmers).
Using a skewer, mark the level of the cider. Continue to mark the level on the skewer each hour.
Simmer the cider for about 3 hours, stirring every 30 minutes. During the last hour of simmering, stir every 15 minutes. (see Recipe Notes)
The cider will be ready when you stir it and dark copper-colored bubbles form, covering the entire surface. You can also check by using your skewer...the cider will boil down to about 1/8 of its original volume. Finally, if you smear a small amount on a plate, it should have the consistency of warm, runny honey. The cider will thicken as it cools, so don't over-cook it.
Once you've achieved the right consistency, remove the boiled cider from the heat and strain it through a fine mesh sieve into a mason jar to remove any impurities. You should have about 1 cup of boiled cider.
Let the jar cool to room temperature, then cover and store in the refrigerator. It should keep indefinately.